This invention relates generally to analog to digital converters and, more specifically, to electro-optic analog-to-digital converters.
Heretofore electronic analog-to-digital converters have been widely used in the industry and may be of the type described by D. F. Hoeschele, Jr. in the text entitled, "Analog to Digital, Digital to Analog Conversion Techniques," published by Wiley in 1968. These are widely used in present day technology to translate sensor measurements of an analog nature into digital language for computing and data processing. However, the maximum speed of operation or output data rate of a typical electronic A/D converter is on the order of 100 megahertz, while the present invention will allow data rates in the gigahertz range.
The use of electro-optic grating type of light beam deflectors for analog to digital conversion was reported by S. Wright, I. M. Mason, and M. G. F. Wilson in an article entitled, "High Speed Electro Optic Analogue-Digital Conversion," published in Electronics Letters, Vol. 10, pp 508-509, Nov. 28, 1974. The grating light beam deflector type of device offers the desirable potential of high speed operation, but unfortunately is inherently limited to the maximum precision of only 3 bits of binary representation.
H. F. Taylor in U.S. Pat. No. 4,058,722 for "Electro Optic Analog/Digital Converter" issued Nov. 15, 1977, has described what is considered to be the most relevant prior art in the field. This patent discloses using a plurality of single dielectric channel optical waveguides fabricated in a single crystal of electro-optic material. Electrodes are disposed on the crystal substrate adjacent to each optical waveguide, for impressing electrical fields thereacross. Linearly polarized laser light energy is applied to each waveguide and an analog signal is applied to the electrodes. The application of the analog signal causes a phase shift and resultant change of polarization in the laser energy which can be detected as a representation of a binary 1 or binary 0 for each of the waveguides. The resultant binary output is the multiple bit analog-to-digital conversion of the analog signal impressed upon the converter.
This technique, however, requires that a polarizing beam splitter be used at the output prior to the comparator and this prior art A/D converter is slower than the A/D converter of the present invention. The beamsplitter is required because the polarization state of the laser energy determines the digitization of the applied analog signal. Thus the two orthogonal components of the energy must be separated to provide this digitization. Additionally, the single waveguide system presumes that the two orthogonal polarizations which define the 0 and 1 binary representation have equal velocity of propagation in the waveguide. However, no isotropic guide has been developed yet.
Thus, it is an object of the present invention to provide an analog to digital converter which operates at a very high speed.
It is a further object of the present invention to provide an electro-optic A/D converter which is not dependent upon the waveguides being isotropic.
Yet another object is to provide an electro-optic A/D converter which is not dependent upon the polarization of the applied laser energy.